While the invention is subject to a number of different applications, it is especially suitable for use in telephone circuits which are used for the transmission of broadcast programs.
In many communications systems, supervisory circuits pass through the same facility as the desired intelligence. For example, in a telephone system certain tones may be sent over the telephone line so as to control various characteristics.
One important example of such operation is in normal dial telephone systems where a 2,600 Hz tone is sent through the line to disconnect the circuit. Unfortunately, certain types of voice and musical waves may confuse such circuitry and falsely operate the disconnect circuit. Sibilant sounds, such as S's and H's produce appreciable energy at 2,600 Hz which is a common frequency used for the line disconnect tone channel. For example, there have been reports of people in the clothing industry whose voices have repeatedly disconnected telephone circuits when they use the term "short sleeve shirts".
Also, there are systems that may be used over communications circuits which alter the spectral characteristics of speech, music and data and which may exasperate the problem. For example, some designs of devices described in U.S. Pat. No. 3,696,298 Kahn and Gordon, U.S. Pat. No. 3,684,838 Kahn, or my copending patent application Ser. No. 621,669 filed Oct. 14, 1975 may, under some conditions, increase the possibility of false operation of in-band signaling devices.
It is most important that when a telephone circuit is used for such transmission of news and sporting events, or for that matter any broadcast service, that the circuit not be interrupted or degraded. A sporting event which may be listened to by millions of people if interrupted at the wrong moment can create considerable consternation on the part of the listening public. More importantly, urgent messages regarding public safety should not suffer interruption and should be as reliable as possible. For this reason, special telephone circuits are often made available where the in-band circuit disconnect channelling equipment is removed from the line.
However, there are many instances where such lines are not available at a suitable cost or where it is impractical to make lines available at short notice. Under such situations normal dial lines equipped with in-band tone control circuits are used for transmission of broadcast programs and these lines are susceptible to false operation.
In the instant application means are provided for detecting the presence of speech sounds which imitate the control tone frequency and attenuating these sounds before they falsely operate the in-band signaling equipment. One embodiment of this invention provides a narrowband reject filter circuit which is normally bypassed but may be inserted in series with the telephone circuit when protection is required. This attenuating means is switched rapidly into the system when protection is required so that the tone control circuitry which is being protected does not have sufficient time to falsely operate. For example, if the tone operated in-band signalling system can operate if the proper frequency tone is fed to it for a 50 ms interval the protection circuit disclosed would insert the attenuating circuit into the system whenever components initiating the tone last for longer than say 30 ms. By this procedure, the control circuit would not operate falsely even though the spectrum characteristic of the speech or musical sound was of the proper characteristic to disturb the in-band signaling equipment.
When the speech sound which is capable of causing the false operation disappeared or drops sufficiently in amplitude to cause a false operation, the circuit attenuating the sound is removed so that the overall system frequency response is not degraded.
In some telephone circuits, band reject filters are permanently inserted in the telephone circuit so as to notch out or remove those portions of the main signal which may create false operation. Since these filters are present in the system permanently they cause some degradation of the frequency response of the line.
Also, an article recently appeared in the sound engineering publication "db" February, 1976 pp. 33 to 36 entitled "The Dynamic Control of Sibilant Sounds" by S. L. Silvers of the Telecommunications Section of the United Nations. This article describes a technique for reducing sibilant sounds which may be present for some speakers and which may be exagerated when conventional volume compressors are used. The article describes a compressor with a frequency sensitive control circuit. The gain reduction is applied to the entire range of speech components whenever strong sibilant sounds are present. This avoids roll off of high frequency sounds. While such a technique is suitable for systems utilizing compressors and counteracts their poor sibilant sound performance it does alter the dynamic character of speech and music appreciably. Common carriers attempt to minimize processing of speech sounds because the subscriber should be allowed to select his own processing procedures and the common carrier system should have constant characteristics and with a minimal processing.
Furthermore, the sibilant sound control circuit is effective over a wide range of frequencies and therefore does not greatly attentuate any one specific narrowband of frequencies. Therefore, such a technique would not be fully effective against false activation of a narrow band tone channel.
In an article authored by Messrs. Weaver and Newell entitled "In-Band Single-Frequency Signalling", Bell Systems Technical Journal, November, 1954, pages 1309 to 1330, the authors describe the use of in-band signaling including circuits and methods for minimizing false keying by imitation tone components which are present in speech and other sounds. The authors discuss the following design factors which were employed to make the system relatively insensitive to the imitation signal tones:
1. A guard action whereby the energy is measured for nearly all of the frequencies in the voice band other than the narrowband centered on the signal frequency. This energy, which would be present under normal speech conditions but not when the in-band signal is present, is used to cancel or reduce the effect of an imitation signal component. Therefore, it materially decreases the sensitivity of the system to false control operation.
2. Employment of as narrow a bandwith as practical for the signal selective network. The use of a narrow bandwidth minimizes the possibility of an imitation signal falsely operating the circuit. Bandwidths in the range of 60 to 150 Hz are recommended.
3. The use of volume limiting to maintain a relatively constant level helps minimize loud talking from falsely operating the system.
4. The use of long operate times consistant with signaling requirements. The longer the operate time the less is the probability that normal speech sounds will imitate the signaling.
5. The use of the highest frequency that can be handled by the voice path for in-band signaling. Since the energy of voice or music falls off rapidly at high frequencies if a high frequency is selected for signaling, the imitation energy will be smaller and therefore will be less capable of falsely operating the signaling equipment.
All of these procedures can be used to greatly reduce the false alarm rate. Nevertheless, even when all of these procedures are utilized false alarms can and do occur often enough to be a problem. Furthermore, there are systems which are used with telephone circuits such as the system described in U.S. Pat. No. 3,684,838 where the spectral energy relationships are disturbed and it is possible that higher numbers of false alarms may be experienced.